Modern gas turbine engines may offer fuel flexibility in that both gas and liquid fuels may be used and that the transfer from one fuel to the other may be accomplished while the engine is running at load. These liquid fuels may include distillate oils, light crude, bio-liquid fuels, and the like. The use of a diverse fuel spectrum provides increased operational flexibility, cost control, plant efficiency, and/or improved emissions characteristics.
Depending upon economic factors, geographic location, and/or availability, liquid fuels may be considered as a start-up or a “backup” fuel to the gas fuel. As such, the liquid fuels may not be in constant use. When a fuel flexible gas turbine is operating on a gas fuel, the liquid fuel lines generally remain full of fuel up to about the combustor fuel nozzles within the gas turbine compartment. If this condition exists for an extended period of time, the liquid fuel therein may undergo a process of decomposition and oxidation. This process may form gummy deposits and ultimately solid particles of coke. The decomposition process may be due to or accelerated by the relatively high temperatures surrounding the fuel lines in the gas turbine compartment. Decomposition may start in as little time as a few weeks or less.
When the liquid fuel lines become partially plugged with gummy and/or solid materials due to decomposition, a gas to liquid fuel transfer may fail due to the plugging of check valves, purge valves, and/or liquid fuel nozzles in the combustor. Repair of the plugged fuel lines, check valves, purge valves, and/or fuel nozzles may require a system outage and a complete teardown of the liquid fuel system for cleanup and replacement of the damaged and/or non-repairable components.
In order to avoid decomposition issues, relatively frequent gas to liquid and liquid to gas fuel transfers may be required. Other alternatives include the use of a liquid fuel recirculation system in which the liquid fuel is continuously recirculated through the liquid fuel lines via the use of special valves at each combustor. The liquid fuel thus may be maintained at a low enough temperature to prevent coking by being recirculated outside of the gas turbine compartment. Such recirculation, however, is somewhat complex and costly given attendant high operating and maintenance costs and also involves some parasitic pump loses. Similarly, the liquid fuel also may be removed from the liquid fuel lines by using a supply of a stored inert gas such as nitrogen to purge the liquid fuel lines back to regions where the temperature surrounding the liquid fuel lines are low enough to avoid decomposition. Such a system also involves complex and costly valves, control sequencing, and the cost of maintaining an adequate supply of nitrogen purge gas.
There is therefore a desire for an improved liquid fuel storage system. Such a system may provide a flow of liquid fuel on demand but also avoid decomposition issues caused by high temperatures and the like while limiting costs, complexity, and parasitic losses.